Fabrication Of Polyoxometalated-Based Artificial Enzymes For Colorimetric Detection Of Sarcosine And Hydrogen Peroxide

Polyoxometalates（POMs）,a kind of metal-oxo-cluster,had been reported to mimic the enzyme activity and named as artificial enzymes.Several tungsten-containing POMs（W-POMs）were found to exhibit good peroxidase-like activity in colorimetric assay of H₂O₂,glucose,or dopamine.Herein,we replaced the W-POMs by Mo/V containing POMs（Mo/V-POMs）,which might provide some useful information for these compounds in the field.Moreover,the biological molecule of sarcosine was selected as a target molecule to check the enzyme-like activity for Mo/V POMs,which was regarded as a biomarker of prostate cancer（PCa）.The findings are:（1）Mo/V-POMs of H_xPM_12-xV_xO₄₀（x=1-3）and（NH₄）₅H₆PM₄V₈O₄₀ was firstly evaluated of their activity in peroxidase-like activity using 3,3’,5,5’-tetramethylbenzidine（TMB）as a substrate.It was found that their peroxidase-like activity was coherent with their redox ability.（NH₄）₅H₆PMo₄V₈O₄₀ was found to be the most active one.（2）The folate（FA）-functionalized POMs（FA-PMo₄V₈）were fabricated using FA and（NH₄）₅H₆PM₄V₈O₄₀ as precursors via self-assembly method.It was characterized by some physical measurement,which determined it keeping Keggin structure.FA-PMo₄V₈exhibited higher peroxidase-like activity with K_m as 2.1×10^-3 m M,which could detect H₂O₂ with a linear range from 0.07 to 4μM and the detection limit 0.012μM in presence of TMB.Most important was that FA-PMo₄V₈acted as sarcosine oxidase（SOD）to catalyze sarcosine reaction with O₂ to generate H₂O₂,glycine and formaldehyde.Hence,sarcosine was detected indirectly via the assay of H₂O₂concentration,which the linear range was 0.2 to 500μM and the detection limit of 0.131μM.FA-PMo₄V₈presented a good selectivity and sensitivity for sarcosine detection in the broad p H range,which was also efficient in sarcosine detection for PCa patients and healthy persons.（3）The solid POMs of PMO₄V₈/chitosan（PMo₄V₈/CS（n））had been fabricated using chitosan as a support（n represented the loading amount）.The hybrids were characterized by some physical measurement to determine the Keggin structure being kept intact.The peroxidase-like activity was measured giving the K_mvalue 4.99 x 10^-4m M for PMo₄V₈/CS（n）.PMo₄V₈/CS（n）showed good enzyme affinity to H₂O₂,which was originated from the synergistic effect between POMs and chitosan.The linear range was 0.25 to 100μM with detection limits of 0.23μM.In sarcosine detection linear range was 0.5-300μM and the detection limit was 0.48μM with an acceptable,precision and recovery.（4）A series of Ag_x（NH₄）₅H_6-xPMo₄V₈（x=1,3,4 and 6）（Ag_xPMo₄V₈）had been synthesized to solidify homogeneous（NH₄）₅H_6-xPMo₄V₈,which was characterized to be Keggin-type POMs.Introduction of Ag⁺into PMo₄V₈ resulted in enhancement in peroxidase-like activity,which was due to the influence of Ag⁺on the redox ability of Ag_xPMo₄V₈.Ag₆PMo₄V₈was found to be most active to give the linear range of 0.05to 4μM and a detection limit 0.009μM for H₂O₂ detection,while K_m=value reached1.28 x 10^-4 m M（R²=0.9950）and 1.1×10^-3 m M（R²=0.9979）corresponding to TMB and H₂O₂.Ag_xPMo₄V₈ could detect sarcosine in the linear range of 0.52-500μM at detection limit of 0.51μM.